1. Field of the Invention
This invention relates to a vaccine for foot-and-mouth disease and more particularly to a vaccine which contains no intact nucleic acid molecules and in which the immunogenic substance is a purified subunit or capsid protein of a foot-and-mouth disease virus (FMDV).
2. Description of the Prior Art
FMDV is an acid-sensitive picornavirus that contains primarily four capsid proteins: VP.sub.1, VP.sub.2, VP.sub.3, and VP.sub.4, as well as a trace of precursor protein VP.sub.o. The capsid proteins of FMDV collectively protect the viral ribonucleic acid core against environmental ribonuclease and other inactivating agents. However, the protein coat of FMDV is not as resistant to proteases as the coats of most enteroviruses. For example, in early experiments on the purification of FMDV, digestions with trypsin led to a marked reduction in infectivity without a corresponding loss in the number of virus particles.
This effect was later found to be caused by the tryptic cleavage of one of the capsid proteins, VP.sub.3, into two peptides, VP.sub.3a and VP.sub.3b, which remained in the capsid structure.
Although these capsid proteins of FMDV are known in the art and have been previously isolated (Biochem. Biophys. Res. Commun., 48, 1222, 1972; J. Gen. Virol., 4, 313, 1969; C. R. Acad. Sci. 276, 3399, 1973), no one, prior to this invention, has discovered that a capsid protein of FMDV could be used as the immunogenic portion of a vaccine to protect animals against exposure to infectious virus.
Since the vaccine of this invention contains no intact nucleic acid molecules, it eliminates the problem found in prior art vaccines of accidental infection because of insufficiently inactivated or modified nucleic acid. This invention also paves the way for synthesis of the immogenic protein or of that part of it needed to produce immunization, thereby eliminating the need for growing huge quantities of virus for vaccine, thus diminishing foci for introducing virus into animals in the field.
Practical vaccines in general use contain virus and cellular debris which can cause sensitization of animals receiving booster doses of vaccine and lead in some cases to the incapacitation or destruction of animals. Since the immunogenic protein of this invention is a pure protein, it reduces to an irreducible minimum the chance of sensitization of animals being vaccinated.